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    <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Debugging</b></font>
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<p>The top-down nature of Spirit makes the generated parser easy to micro- debug
  using the standard debugger bundled with the C++ compiler we are using. With
  recursive-descent, the parse traversal utilizes the hardware stack through C++
  function call mechanisms. There are no difficult to debug tables or state machines
  that obscure the parsing logic flow. The stack trace we see in the debugger
  follows faithfully the hierarchical grammar structure.</p>
<p> Since any production rule can initiate a parse traversal , it is a lot easier
  to pinpoint the bugs by focusing on one or a few rules. For relatively complex
  parsing tasks, the same way we write robust C++ programs, it is advisable to
  develop a grammar iteratively on a per-module basis where each module is a small
  subset of the complete grammar. That way, we can stress-test individual modules
  piecemeal until we reach the top-most module. For instance, when developing
  a scripting language, we can start with expressions, then move on to statements,
  then functions, upwards until we have a complete grammar. </p>
<p> At some point when the grammar gets quite complicated, it is desirable to
  visualize the parse traversal and see what's happening. There are some facilities
  in the framework that aid in the visualisation of the parse traversal for the
  purpose of debugging. The following macros enable these features.</p>
<a name="debugging_macros"></a>
<h2>Debugging Macros</h2>
<a name="spirit_assert_exception"></a>
<h3>BOOST_SPIRIT_ASSERT_EXCEPTION</h3>
<p> Spirit contains assertions that may activate when spirit is used incorrectly.
  By default these assertions use the assert macro from the standard library.
  If you want spirit to throw an exception instead, define <tt>BOOST_SPIRIT_ASSERT_EXCEPTION</tt>
  to the name of the class that you want to be thrown. This class's constructor
  will be passed a <tt>const char*</tt> stringified version of the file, line,
  and assertion condition, when it is thrown. If you want to totally disable the
  assertion, <tt>#define NDEBUG</tt>.</p>
<a name="spirit_debug"></a>
<h3>BOOST_SPIRIT_DEBUG</h3>
<p>Define this to enable debugging.</p>
<p>With debugging enabled, special output is generated at key points of the
  parse process, using the standard output operator (<tt><span class="keyword">operator</span><span class="special">&lt;&lt;</span></tt>)
  with <tt>BOOST_SPIRIT_DEBUG_OUT</tt> (default is <tt><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span></tt>,
  see below) as its left operand.</p>
<table width="80%" border="0" align="center">
  <tr>
    <td class="note_box"><img src="theme/note.gif"> In order to use spirit's
    debugging support you must ensure that appropriate overloads of 
    <tt><span class="identifier">operator</span><span class="special">&lt;&lt;</span></tt>
    taking <tt>BOOST_SPIRIT_DEBUG_OUT</tt> as its left operand are available.
    The expected semantics are those of the standard output operator.<br>
    <br>
    These overloads may be provided either within the namespace where the
    corresponding class is declared (will be found through Argument Dependent Lookup) or [within an
    anonymous namespace] within <tt><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span></tt>,
    so it is visible where it is called.<br>
    <br>
    <img src="theme/alert.gif"> Note in particular that when <tt>BOOST_SPIRIT_DEBUG_FLAGS_CLOSURES</tt>
    is set, overloads of <tt><span class="identifier">operator</span><span class="special">&lt;&lt;</span></tt>
    taking instances of the types used in closures as their right operands are required.<br>
    <br>
    You may find an example of overloading the output operator for
    <tt><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></tt>
    in a <a href="faq.html#output_operator">related FAQ entry</a>.</td>
  </tr>
</table>

<p>By default, if the <tt>BOOST_SPIRIT_DEBUG</tt> macro is defined, all available
  debug output is generated. To fine tune the amount of generated text you can
  define the <tt>BOOST_SPIRIT_DEBUG_FLAGS</tt> constant to be equal of a combination
  of the following flags:</p>
<table width="90%" border="0" align="center">
  <tr>
    <td colspan="2" class="table_title"><b>Available flags to fine tune debug
      output </b></td>
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  <tr>
    <td width="29%" height="27" class="table_cells"><tt>BOOST_SPIRIT_DEBUG_FLAGS_NODES</tt></td>
    <td width="71%" class="table_cells"><p>print information about nodes (general
        for all parsers)</p></td>
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  <tr>
    <td height="27" class="table_cells"><tt>BOOST_SPIRIT_DEBUG_FLAGS_TREES</tt></td>
    <td class="table_cells"><p>print information about parse trees and AST's (general
        for all tree parsers)</p></td>
  </tr>
  <tr>
    <td height="27" class="table_cells"><tt>BOOST_SPIRIT_DEBUG_FLAGS_CLOSURES</tt></td>
    <td class="table_cells">print information about closures (general for all
      parsers with closures)</td>
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  <tr>
    <td height="27" class="table_cells"><tt>BOOST_SPIRIT_DEBUG_FLAGS_ESCAPE_CHAR</tt></td>
    <td class="table_cells"><p>print information out of the <tt>esc_char_parser</tt></p></td>
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    <td height="27" class="table_cells"><tt>BOOST_SPIRIT_DEBUG_FLAGS_SLEX</tt></td>
    <td class="table_cells">print information out of the <tt>SLEX</tt> parser</td>
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<p><a name="spirit_debug_out"></a> </p>
<h3>BOOST_SPIRIT_DEBUG_OUT</h3>
<p> Define this to redirect the debugging diagnostics printout to somewhere else
  (e.g. a file or stream). Defaults to <tt>std::cout</tt>.</p>
<a name="spirit_debug_token printer"></a>
<h3>BOOST_SPIRIT_DEBUG_TOKEN_PRINTER</h3>
<p> The <tt>BOOST_SPIRIT_DEBUG_TOKEN_PRINTER</tt> macro allows you to redefine the way characters are printed on the stream. </p>
<p>If <tt>BOOST_SPIRIT_DEBUG_OUT</tt> is of type <tt>StreamT</tt>,  the character type is <tt>CharT</tt> and <tt>BOOST_SPIRIT_DEBUG_TOKEN_PRINTER</tt> is
  defined to <tt>foo</tt>, it must be compatible with this usage:</p>
<pre><code><span class=identifier>    foo</span><span class=special>(</span><span class=identifier>StreamT</span><span class=special>, </span><span class=identifier>CharT</span><span class=special>)</span></code></pre>
<p>The default printer requires <tt>operator&lt;&lt;(StreamT, CharT)</tt>  to
  be defined. Additionally, if <tt>CharT</tt> is convertible to a normal character
  type (<tt>char</tt>, <tt>wchar_t</tt> or <tt>int</tt>), it prints control
  characters in a friendly manner (e.g., when it receives <span class=special>'\n'</span> it
  actually prints the <span class=special>\</span> and <span class=special>n</span> characters,
instead of a newline).</p>
<a name="spirit_debug_print_some"></a>
<h3>BOOST_SPIRIT_DEBUG_PRINT_SOME</h3>
<p> The <tt>BOOST_SPIRIT_DEBUG_PRINT_SOME</tt> constant defines the number of
  characters from the stream to be printed for diagnosis. This defaults to the
  first 20 characters.</p>
<p><a name="spirit_debug_tracenode"></a> </p>
<h3>BOOST_SPIRIT_DEBUG_TRACENODE</h3>
<p> By default all parser nodes are traced. This constant may be used to redefine
  this default. If this is <tt>1</tt> (<tt>true</tt>), then tracing is enabled
  by default, if this constant is <tt>0</tt> (<tt>false</tt>), the tracing is
  disabled by default. This preprocessor constant is set to <tt>1 </tt>(<tt>true</tt>)
  by default.</p>
<p>Please note, that the following <tt>BOOST_SPIRIT_DEBUG_...() </tt>macros are
  to be used at function scope only.</p>
<a name="spirit_debug_node_p_"></a>
<h3>BOOST_SPIRIT_DEBUG_NODE(p)</h3>
<p> Define this to print some debugging diagnostics for parser p. This macro</p>
<ul>
  <li>Registers the parser name for debugging</li>
  <li>Enables/disables the tracing for parser depending on <tt>BOOST_SPIRIT_DEBUG_TRACENODE</tt></li>
</ul>
<p> <b>Pre-parse</b>: Before entering the rule, the rule name followed by a peek
  into the data at the current iterator position is printed.</p>
<p> <b>Post-parse</b>: After parsing the rule, the rule name followed by a peek
  into the data at the current iterator position is printed. Here, <tt>'/'</tt>
  before the rule name flags a successful match while <tt>'#'</tt> before the rule
  name flags an unsuccessful match.</p>
<p> The following are synonyms for <tt>BOOST_SPIRIT_DEBUG_NODE</tt></p>
<ol>
  <li>BOOST_SPIRIT_DEBUG_RULE</li>
  <li>BOOST_SPIRIT_DEBUG_GRAMMAR</li>
</ol>
<a name="spirit_trace_node_p__flag_"></a>
<h3>BOOST_SPIRIT_DEBUG_TRACE_NODE(p, flag)</h3>
<p> Similar to <tt>BOOST_SPIRIT_DEBUG_NODE</tt>. Additionally allows selective debugging.
  This is useful in situations where we want to debug just a hand picked set of
  nodes.</p>
<p> The following are synonyms for <tt>BOOST_SPIRIT_DEBUG_TRACE_NODE</tt></p>
<ol>
  <li>BOOST_SPIRIT_DEBUG_TRACE_RULE</li>
  <li>BOOST_SPIRIT_DEBUG_TRACE_GRAMMAR</li>
</ol>
<p><a name="spirit_trace_node_p__flag__name_"></a> </p>
<h3>BOOST_SPIRIT_DEBUG_TRACE_NODE_NAME(p, name, flag)</h3>
<p> Similar to <tt>BOOST_SPIRIT_DEBUG_NODE</tt>. Additionally allows selective
  debugging and allows to specify the name used during debug printout. This is
  useful in situations where we want to debug just a hand picked set of nodes.
  The <tt>name</tt> may be redefined in situations, where the parser parameter does not reflect the name of the parser to debug.</p>
<p> The following are synonyms for <tt>BOOST_SPIRIT_DEBUG_TRACE_NODE</tt></p>
<ol>
  <li>BOOST_SPIRIT_DEBUG_TRACE_RULE_NAME</li>
  <li>BOOST_SPIRIT_DEBUG_TRACE_GRAMMAR_NAME</li>
</ol>
<hr>
<p>Here's the original calculator with debugging features enabled:</p>
<pre>
    <code><span class=preprocessor>#define </span><span class=identifier>BOOST_SPIRIT_DEBUG  </span><span class=comment>///$$$ DEFINE THIS BEFORE ANYTHING ELSE $$$///
    </span><span class=preprocessor>#include </span><span class=string>&quot;boost/spirit/include/classic.hpp&quot;

    </span><span class=comment>/***/

    /*** CALCULATOR GRAMMAR DEFINITIONS HERE ***/

    </span><span class=identifier>BOOST_SPIRIT_DEBUG_RULE</span><span class=special>(</span><span class=identifier>integer</span><span class=special>);
    </span><span class=identifier>BOOST_SPIRIT_DEBUG_RULE</span><span class=special>(</span><span class=identifier>group</span><span class=special>);
    </span><span class=identifier>BOOST_SPIRIT_DEBUG_RULE</span><span class=special>(</span><span class=identifier>factor</span><span class=special>);
    </span><span class=identifier>BOOST_SPIRIT_DEBUG_RULE</span><span class=special>(</span><span class=identifier>term</span><span class=special>);
    </span><span class=identifier>BOOST_SPIRIT_DEBUG_RULE</span><span class=special>(</span><span class=identifier>expr</span><span class=special>);
</span></code></pre>
<p> <img src="theme/note.gif" width="16" height="16"> Be sure to add the macros <strong>inside</strong> the grammar definition's constructor. Now here's a sample session with the calculator.</p>

<pre><code>    <span class="preprocessor">Type an expression...or [q or Q] to quit</span>

    <span class="preprocessor">1 + 2</span>

    grammar(calc):	"1 + 2"
      rule(expression):	"1 + 2"
        rule(term):	"1 + 2"
          rule(factor):	"1 + 2"
            rule(integer):	"1 + 2"
    <span class="preprocessor">push	1</span>
            /rule(integer):	" + 2"
          /rule(factor):	" + 2"
        /rule(term):	" + 2"
        rule(term):	"2"
          rule(factor):	"2"
            rule(integer):	"2"
    <span class="preprocessor">push	2</span>
            /rule(integer):	""
          /rule(factor):	""
        /rule(term):	""
    <span class="preprocessor">popped 1 and 2 from the stack. pushing 3 onto the stack.</span>
      /rule(expression):	""
    /grammar(calc):	""
    <span class="preprocessor">-------------------------
    Parsing succeeded
    result = 3
    -------------------------</span></code></pre>

<p> We typed in &quot;1 + 2&quot;. Notice that there are two successful branches
  from the top rule <tt>expr</tt>. The text in red is generated by the parser's
  semantic actions while the others are generated by the debug-diagnostics of
  our rules. Notice how the first <tt>integer</tt> rule took &quot;1&quot;, the
  first <tt>term</tt> rule took &quot;+&quot; and finally the second <tt>integer</tt>
  rule took &quot;2&quot;.</p>
<p>Please note the special meaning of the first characters appearing on the printed
  lines:</p>
<ul>
  <li>a single <span class="literal">'/'</span> starts a line containing the information
    about a successfully matched parser node (<tt>rule&lt;&gt;</tt>, <tt>grammar&lt;&gt;</tt>
    or <tt>subrule&lt;&gt;</tt>)</li>
  <li>a single <span class="literal">'#'</span> starts a line containing the information
    about a failed parser node</li>
  <li>a single <span class="literal">'^'</span> starts a line containing the first member (return value/synthesised
    attribute) of the closure of a successfully matched parser node.</li>
</ul>
<p>Check out  <a href="../example/fundamental/calc_debug.cpp">calc_debug.cpp</a> to see debugging in action. </p>
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